The present invention relates to an optical high-pressure transmission cell, preferably a measuring cell for SFC-FTIR, comprising a pressure-resistant housing element which is passed by two bores of different diameters extending perpendicularly relative to, and intersecting, each other, a thinner bore forming a passage for a highly compressed solvent containing a substance under examination, whereas a thicker bore forms a transmission path for light of an IR spectrometer, further fittings for connection of lines to ends of the thinner bore and IR-permeable windows closing off the thicker bore on both sides at points of opening of the thinner bore, each of the said windows being arranged in a recess provided in the housing part at an end of the thicker bore and being held in the said recess by a clamping body resting against an outer surface of the window and fixed to the housing part by screwing, and being sealed off relative to the housing part by means of an O-ring arranged between the housing part and the inner window surface adjoining the housing part, so that the thicker bore delimited by the windows arranged opposite each other defines a high-pressure resistant, tight chamber for the agent, or solvent containing substance passed therethrough by means of the thinner bore.
A measuring cell of this type has been known before from U.S. Pat. No. 4,588,893. A housing element of the known measuring cell comprises two plane surfaces, and recesses for receiving windows are arranged in areas of such surfaces. Outer portions of the windows extend beyond the plane surfaces and are retained in position by annular clamping bodies which are fastened to the housing element by threaded bolts. O-rings serving to seal off the windows are accommodated in annular grooves provided in bottoms of the recesses, surrounding the thicker bore in the form of a ring. The fact that the grooves for the O-rings are arranged opposite each other makes it necessary to give the housing element a considerable thickness which also determines a length of the thicker bore between the windows and, consequently, a length of a path of IR radiation through the chamber defined by the windows. In the case of the embodiment described by U.S. Pat. No. 4,588,893, the thicker bore has a length of five millimeters. In addition, the known measuring cell has a light tube inserted in this bore for establishing a connection to the opening of the thinner bore through openings provided in its periphery so that light is passed by an agent under examination in a direction perpendicular to the windows. It is another drawback of the known measuring cell that uniform tightening of the threaded bolts serving for fastening clamping bodies is very difficult because distortions may easily occur and will then stress the IR-permeable windows heavily. On the one hand, these windows are very delicate physically, so that they may easily be destroyed when stressed improperly. On the other hand, even slight distortions may change optical properties of the windows and, consequently, impair quality of IR-spectrometric measurements.
Another previously known measuring cell for SFC-FTIR is marketed by the AABSPEC companies in the U.S.A. and in Ireland under the name "MODEL 4000". According to a description given for these measuring cells, windows are arranged in a tubular body provided, in its central area, with an internal shoulder supporting the windows, preferably with a Teflon ring inserted between the windows and the shoulder. Sealing as such is effected by means of O-rings which rest against outside surfaces of the windows and which, consequently, are arranged between the windows and clamping bodies holding the windows. The exact design of these clamping bodies cannot be derived from the description of this measuring cell. However, the description warns the user not to load a central shoulder unevenly or to stress the windows excessively as this might lead to distortions and eventually to breakage of the windows. According to the description, the minimum distance achievable in this manner between the windows is equal to 0.5 mm, which distance can be increased up to 20 mm by means of spacer rings.
For a measuring cell known from U.S. Pat. No. 4,588,893, a typical pressure of a supercritical solvent is said to be between 70 and 140 bar so that it may be assumed that this measuring cell is designed for a maximum pressure of up to approx. 150 bar. As compared to this, the AABSPEC cell is intended to stand a pressure of up to approx. 280 bar (4000 psi). In fact, in cases of known measuring cells, disturbing distortions or even damage might result not only from improper clamping of windows, but also from an excessively high fluid pressure loading the windows.
Stepped windows have also been known in the prior art; this appears, for example, from DE-28 10 388 A1, DE No. 28 42 367 A1, British journal J. Sci. Instrum. 44, 1967, pages 876 to 878, and German journal Chem. 1, 1961, pages 234 to 245.
Only the two last-mentioned publications describe high-pressure measuring cells, and the purpose of the axially stepped windows illustrated in FIG. 24 on pages 241 and 242 of German journal Chem. 1, 1961 consists in reducing window openings to very small dimensions, in an area of a sample, so that crystals used for the windows, for example NaCl, can withstand the very high pressures encountered.
In contrast, DE No. 28 10 388 A1 describes a device for measuring by optical means a preselected property of a substance flowing through a line, while DE No. 28 42 367 A1 describes a device for introducing particles into analyzers for the purpose of analyzing dust content of gases.
In view of progress achieved in chromatography with supercritical phases (Supercritical Fluid Chromatography SFC) in connection with Fourier Transform IR Spectroscopy (FTIR) it would seem desirable to have a measuring cell capable of being operated at pressures of supercritical solvents of 400 bars and even more, without any risk of destruction of windows by distortions or disturbance of transmission of a light beam used for IR spectroscopy. At the same time, it is desired to have a very short light path through fluids.